1. Field of the Invention
The present invention relates to a method of manufacturing castings having a unidirectional solidification structure of a metal active in the molten state or an alloy thereof (hereinafter referred to as "active metal") such as Ti or a Ti alloy.
2. Prior Art
The method of manufacturing castings having a unidirectional structure such as columnar-crystal or single-crystal castings comprises in general, as shown in FIG. 2, heating a metal to a temperature above the melting point thereof in a high-vacuum atmosphere, teeming molten metal 10 into a mold 3 placed on a water-cooled copper cooling plate 5, and preventing solidification of molten metal 10 by surrounding the mold 3 with a graphite heating element 2 heated by an induction coil 1'.
Cooling the water-cooled copper cooling plate 5 in this state causes columnar crystal grains 9 to be produced upward from said water-cooled copper cooling plate 5 and grow in a single direction while solidifying under the effect of descent of the water-cooled copper cooling plate.
The molten metal 10 charged for manufacturing castings having a unidirectional structure is held at a temperature for a long period of time in the molten state in the mold 3. If said molten metal is a metal active in molten state such as Ti or a Ti alloy, said active metal would erode the mold 3, causing such problems as the reaction with impurities coming from the mold and roughening of the casting surface.